Method for making prestressed precast concrete elements

ABSTRACT

Process for the production of prestressed precast concrete elements comprises tensioning high-tensile steel tendons to a predetermined degree of elongation, casting a section of concrete of predetermined length along a certain section of the tendon, allowing this to harden, decreasing the tensioning of the tendon by a predetermined decrement, casting a further section of concrete in continuation of one or both sides of the first cast section, and if desired, repeating this process of further decreasing of tensioning and casting further sections.

`Fume 25, 1974 z. KmRoN l 3,819,794 METHOD FOR MAKING PRESTRESSED PRECAST CONCRETE ELEMENTS I 2 Sheets-Sheet l v Filed May l5, 1972 fg NV M .d i

5 5 5 ^E moz@ .N .a u l S @o m .ai l 1 u Hu u HTA, U 1 u Q VU U 6 u Z. KIDRON l `lune 25, 1974 METHOD FOR MAKING PRESTRESSED PRECST CONCRETE ELEMENTS 2 Sheets-Sheet 2 Filed. May l5., 1972 U.S. Cl. 264-228 United States Patent Oice 3,819,794 Patented June 25, 1974 3,819,794 METHOD FOR MAKING PRESTRESSED PRECAST CONCRETE ELEMENTS Zeev Kidron, 20 Simtat Iris, Ramat-Hasharon, Israel Filed May 15, 1972, Ser. No. 253,208 Int. Cl. B28b 23/04 6 Claims ABSTRACT F THE DISCLOSURE Process for the production of prestressed precast concrete elements comprises tensioning high-tensile steel tendons to a predetermined degree of elongation, casting a section of concrete of predetermined length along a certain section of the tendon, allowing this to harden, decreasing the tensioning of the tendon by a predetermined decrement, casting a further section of concrete in continuation of one or both sides of the first cast section, and if desired, repeating this process of further decreasing of tensioning and casting further sections.

BACKGROUND OF THE INVENTION The present invention relates to an improved method for preparing prestressed concrete elements, and resides mainly, but not exclusively, in the steps of pretensioning the steel reinforcements to be incorporated in the said elements.

The usual method of manufacturing prestressed beams or other elements according to the pretensioning process, consists in tensioning the high-tensile steel tendons-either wires or seven-wire strands--in the prestressing bed prior to casting the concrete of the elements (FIG. 1). Thus, upon release of the tendons after hardening of the concrete, the tensile force in the tendons is transferred from the abutments to the concrete elements, which are brought into a state or nearly permanent compression.

As a rule, the tendons are placed excentrically with respect to the center of gravity of the cross-section of the concrete elements-as near as possible to the bottom face of the beam or element. This has the advantage that less high-tensile steel is needed to get high compressive stresses near the critical bottom fibre.

The maximum possible excentricity is limited by the requirement not to produce transverse cracks emanating from the upper face, due to prestress. The effects of the prestress are immediately counteracted by the weight of the manufactured elements: indeed, upon release of tendons and transfer of stresses to the concrete, the concrete elements assume a cambered shape, lifting themselves from the stressing-bed (FIG. 2). Bending moments due to the weight of the beams have a tendency to close the cracks which open at the top face.

In every prestressed concrete design, the dead load is consistently taken into account, when considering the maximum allowable excentricity of the tendons. By bending due to dead load varies from zero at the ends to maximum at the center of the element, so that if the benelicial effect of the weight is made use of at the utmost at the center of the elements, nevertheless cracks would open near the extremities, where the effect of bending due to dead load is absent.

Two known procedures, in widespread use, eliininate the danger of cracking at the upper face near the extremities of the elements:

(A) Draped or harped tendons: in this method, part or the totality of tendons are deected upwards, by means of hold-down points, thus reducing the excentricity near the ends of the element (FIG. 3).

(B) Blanketed tendons: in this method, bond to the concrete is prevented for some of the pretensioned tendons by means of plastic tubing which surrounds the blanketed tendons throughout certain lengths measured from the ends of the element. The transfer of stresses from the tendon to the concrete is obtained near the ends of the blanketed section, so that the prestress level at the ends of the element may be reduced.

The above-mentioned methods have various drawbacks:

The draping of the tendons is a costly and time-consuming fabrication process. Release of prestress is complicated and additional cracking may appear above holddown points.

Blanketing impairs bond between tendons and concrete and may result in a premature fracture at ultimate load. (Ref.: Paul H. Kaar & Donald D. Majura: effect of strand blanketing on performance of pretensioned girders, P.C.I. Journal, December 1965, vol. 10No. 6.)

BRIEF SUMMARY OF THE INVENTION The present invention provides a method of controlled detensioning of the high-tensile steel tendons, imparting varying prestresses to the steel, and thus eliminates the needs for draping or blanketing the tendons.

According to a broad aspect of the invention there is provided a process for the production of prestressed precast concrete elements, which comprises tensioning hightensile steel tendons to a predetermined degree of elong-ation, casting a section of concrete of predetermined length along a certain section of the tendon, allowing this to harden, decreasing the tensioning of the tendon by a predetermined decrement, and casting a further section of concrete in continuation of one or both sides of the first cast section.

BRIEF DESCRIPTION KOF THE DRAWINGS The invention will now be described with reference to the annexed drawings. In the drawings FIGS. 1-3 relate to the known-art and have already been referred to above. In FIG. 1, the letter a indicates prestressed beam elements which are to be poured on the stressing bed b. The abutments are indicated by the letter c. The reinforcement is constituted, as has been stated above, by wires or cables each of which is indicated by the letter d and is anchored at e. FIG. 2 illustrates in a schematically-and somewhat exaggerated mannerthe shape which such an element assumes after release of the tendons. The central part is lifted upwardly and cracks f appear at the top face. These close under load. FIG. 3 shows in a schematical manner that in the method employing draped tendons, these latter are directed upwardly.

The invention is illustrated by FIGS. 4-8.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will now be described with reference to the annexed drawings which illustrate the invention schematically:

With reference iirst to FIG. 4, assuming that a bridgegirder shall be given an optimum varying prestressingforce-to be determined by the designer-according to the following scheme:

Pf-:P1mm in its middle section, zone I.

P2 P1 in zone Il, extending from the limits of the middle section in both directions up to a point situated at a speciied distance from the ends.

P3 P2 in zone III, extending from either end to the limits of zone II.

In order to arrive at the above prestressing forces in the specified zones, the execution shall proceed as follows:

(a) Tensioning of tendons up to the total maximum prestressing force P1 (see FIG. 5).

(b) Casting of concrete in middle zones I only.

3 A (c) Partial detensioning first decrement AP1=(P1P2), after sufficient hardening of zones I to suffer prestress amount to AP1 at the given excentricity (see FIG. 6).

, (d) Casting of concrete in intermediate zones II.

(e) Additional partial detensioning of tendons by the value of the second decrement AP2= (P2-P3), after sufiicient hardening of zones II to suffer prestress amounting to AP2, and subsequent hardening of zones I to resist adequately the influence of the sum of decrements API-l-APZ, at the given excentricity (see FIG. 7).

y(f) Casting of concrete in end sections, zones III.

. (g) Final release and transfer of prestressing forces after suflcient hardening of all zones to get their respective iinal shares of initial prestress at the given excentricity (see FIG. 8).

The successive increments of the prestressing compressive force in the concrete, corresponding to the contrvolled decrements of the detensioning process, are indicated in FIGS. 5, 6, 7, s.

No specialized equipment for the application of the controlled detensioning process should be required, other than the normal equipment used by pretensioning plants, equipped with apparatus for multiple strand detensioning; the tendons must be released simultaneously by hydraulic jacks. The only special device that might Abe needed is an adjustable buttress, situated symmetrically with respect to the detensioning jacks, necessary to halt the release at the predetermined position, in accordance with the computations of elongation recovery (see following section). This adjustable buttress might be provided by a series of wedges or threaded studs, but equally well by nuts, accommodated on the threaded outer perimeter of the detensioning jacks themselves.

To successmully execute tensioning and detensioning operations, the required initial elongation, as well as the negative elongations, i.e. the back-travel of the anchorageheader during detensioning in stages, must correctly be predetermined. Initial elongation is computed as usual, using data obtained directly from the stress-strain diagram of the tendons, and considering anchorage losses ete. Subsequent decrements in elongation to be obtained for successive partial releases, may be computed by the formula:

APn

ALn decrement in elongation (back-travel of the detensioning jacks), corresponding to a decrease in tensile force from Pn to Pam). y

APn decrease of tensile force in exposed tendon sections from Pn to P(n+1).

As total area of steel cross-section of tendons.

Es Young-modulus, obtained for unloading, from the stress-strain diagrams of the tendons.

2Le sum of lengths of exposed tendon sections between members.

Ecn Young modulus of concrete for unloading, at the age corresponding to detensioning-stage APn.

fc(APn) local compressive stress in concrete, due to prestress-increment APn=Pn-P(n+1), at C. G. of tendon level (after deduction of stress duc to dead load of member).

dLc differential of tendon-length embedded in concrete.

The first term in the right-hand member of above formula, represents the decrement due to exposed tendon lengths, whereas the second term corresponds to the part of `decrement due to the sum of lengths of the embedded tendon sections.

Precautions must be taken to equalize tensile forces in exposed tendon sections between consecutive members after each partial detensioning operation. This will be achieved if no restriction in longitudinal movement is allowed during release, i.e. if sliding friction between bottom of elements and stressing-bed s practically reduced of tendons by a value of the to nil. Use of effective bond-breakers or polyethylene sheets may be advisable to reach this purpose. The strain in every exposed tendon section should be controlled with the aid of strain-gauges or other instrumentation. Measurement of force by load-cells at the abutments only is unsatisfactory; release of tensile forces restrained by bottom friction will not be transferred to the concrete of the adjacent members.

The duration of the manufacturing-cycle on the stressing-bed is governed to a large `extent by the method of concrete curing: moist-curing, heator steam-curing. Indeed, stress transferv cannot be done before the concrete strength, as indicated by test-cylinders,reaches certain specified transfer strengths.

But regardless of which type of curing is being applied, the introduction of the controlled detensioning procedure should in no case retard the manufacturing-cycle. l

The specified transfer strength for a certain detensioning stage is a function of the compressive strength attained at the moment of the release; not of the considerably higher compressive strength arrived at, at the final release. Therefore, for a given curing method with a known strength-versus-time diagram, it may be seen that the earliest possible time of stress-transfer for a certain detensioning stage depends on the ratio Pn/Ptom, where Pn denotes the prestress arrived at, after the given partial detensioning.

In the case of steam curing, the detensioning must be done while the concrete is still warm and moist; thus practically, the phases of detensioning may be executed with little interruption of 'the curing process. Moreover, if steam enclosures are conformed only to that part of members already cast, this will result in economy of energy and will allow the casting of adjacent portions of members in a normal atmosphere.

Any remaining time losses due to possible interruption of curing, will be compensated by the elimination of eventual draping of strands or sheathing operations for blanketed strands.

Some unavoidable side-elects are associated with the controlled detensioning method.

1. Decompressing in concrete, due to restrained creep: The concrete which is put into compression as a result of a certain stage of detensioning, has no possibility to creep in compression, while the tensioned tendons restrain that creep. This may readily be checked by applying the formula given above. If the modulus Ecn is replaced by an apparent lower modulus Ecn, while Ln is kept constant, the concrete compressive stress fc(APn), must decrease correspondingly.

These losses may be detected with the aid of strain gauges applied on the concrete elements, and they can easily be compensated with additional prestress and readjustment of tension in the tendons, or they may bev accounted for, by the designer.

Due to casting in stages and the successive detensioning procedure, every cast section will assume its cambered shape, associated with the last detensioning stage, immediately after every partial release. The superposition of various cambered shapes will result in a broken camber. However, the total camber is reduced, due to the reduction of the prestressing force at the ends. It is expected that'the break in camber will hardly be noticed.

The body-a girder-indicated as a whole by the numeral l is subdivided into an inner zone I, flanked by two zones H and two end zones III. Of course outwardly these five zones are not visibly differentiated from one another.

The stressing bed has at each of its two ends an abutment indicated by the numeral 2. The high tensile steel reinforcement or tendons (the nature of which will become evident from the following practical example) is indicated by the numeral 3.

On the bed (see FIG. 6) three girdersare being made; all three are cast on the tendons 3.

In a first step the initial stress of P1 is applied to the tendons, whereupon the middle zones I are cast. This is followed by partial release of tension, as soon as the concrete has sufliciently set. Now the two flanking zones II are cast on each girder (FIG. 7) and a further partial release takes place, after setting of zones II. Finally the end zones III are cast and total release of tension is effected, after hardening of zones III.

The tensile force acting on each zone is at every stage as indicated in the drawing. The final effect is that zone I had undergone a tensile force P1, zones II-Pz--and zones III-P3 with corresponding force decrement.

In order to make fully clear the nature of the invention and how it is worked in practice, the following practical example is given.

EXAMPLE The following is a practical example illustrating the application of the method, according to the invention. The type of precast member to be made and treated is a girder rn. long, with an I-type cross-section of 57 cm. depth (half-size of type 3 standard AASHO bridge girder). The required maximum initial prestressing force of P1=76 tons at a given excentricity, shall |be obtained by means of y12 high-tensile steel .seven-wire strands, each of 5%" diam. (with a total cross-section of Ag=12 51.5 :620 mm2).

The desired longitudinal distribution of the prestressing force-as fixed by the designer-is as follows:

100% of the maximum prestressing force, in the central zone I of 3.0 length,

75% of the maximum prestressing force, in 2 adjoining sections, each 1.10 m. -long (zone Iii), and

50% of the maximum prestressing force, in the remaining f2 end-sections, each 2.5 m. long (zone I-II) The girders will be manufactured in a stressing bed, 63.5 m. long `between fixed abutments. Thus six girders may be placed between abutments, with 0.5 m. intervals between consecutive girders and 0.5 m. clear space between the first or last girder and the abutment-face.

The manufacture will proceed in the following order:

1. Tensioning and anchoring of 12 high-tensile steel strands of diam. at the abutments at an initial stress of 122.5 kg./mm.2.

Initial force P1=122.5 X 620 mm.2X 76,000 kg.=76 metric tons. Initial elongation 122.5 kga/mm.2 20,500 lig/mmf 2. Casting of 6 middle zones I (1 section per beam).

3. First partial release corresponding to a tensile force decrement of (1.0-0.75)76.0=19.0 tons. This will be obtained by an elongation decrement of Al1=70.5k mm. This first partial release will take place as soon as the cast concrete sections (zones I) have hardened sufiiciently to sustain a partial prestress of 19.0 tons.

4. Casting of zones II (l2 segments, 2 segments per beam).

5. Second partial release corresponding to a tensile force decrement equal to the first decrement of 19.0 tons. This will be obtained by a further elongation decrement of 55.5 mm.

The second partial release will take place as soon as the concrete in zones II have hardened sufficiently to support a prestress of 19.0 tons, and the concrete in middle zones I is able to support a prestress of 19+19 :38 tons.

6. Casting of zones III (12 segments, 2 segments per beam).

7. Final and total release corresponding to the release of the residual force of 38.0 tons, which will be executed as soon as the concrete in zones III is strong enough to support a prestress of 38.0 tons, the concrete in zones II,

6 to support a prestress of 19|38=57 tons, and the concrete in zones I, to support a prestrress of 38+38=76 tons.

Anjuwzffmlndn wherein EL1=exposed tendon length during 4first release =2 x 40 x 5 x 7.50=44.5 m.=44,500 rnm. 2L2=exposed tendon length during second release =2 X 30|5 X 5.50=33.0 m.'=33.500 mm.

and the following additional assumptions have been made:

=ES=20,500 kg./mm.2 Bc1.==Ec2=200,000 kg./mm.2 fc (P1)=fc(P2)=37.5 kg./mm.2-constant along girder.

X6X 3,000=67|3.5=70.5 mm.

XGX 5,000= 50+ 5.6: 55.6 mm.

ALI

What is claimed is:

1. Process for the production of a prestressed precast concrete element having a predetermined longitudinal axis, which `comprises positioning a plurality of hightensile steel tendons at a location of substantially uniform excentricity with respect to said predetermined longitudinal axis as near as possible to the bottom face of the element, tensioning said high-tensile steel tendons to a first predetermined degree of elongation having a predetermined maximum prestressing force, casting a first section of concrete of predetermined length about a predetermined longitudinal section of the tendons, allowing said 'first concrete section to harden, decreasing the tensioning of the tendons by a first predetermined decrement after said first concrete section has hardened enough to retairi" an initial prestress of said first decrement, casting a second section of concrete about the tendons longitudinally adjacent to at least one side of the first cast `section in continuation thereof, allowing said concrete section to further harden and further decreasing the tensioning of the tendons by a second predetermined decrement after said second section has hardened enough to retain an initial prestress of said second decrement and said first section has hardened enough to retain a first subsequent prestress of the sum of said first and second decrements, whereby the value of the tendon forces in said prestressed concrete element is progressively controllably reduced from said first section to said second section and cracks in said concrete element due to said prestressing are minimized.

2. A process as claimed in claim 1, wherein casting of the element comprises casting one central section as said first section and subsequently casting two adjacent sections fianking said central section as continuation thereof as said second section, whereby the value of the tendon forces from the zone of maximum bending in said prestressed concrete element towards the longitudinal extremities thereof is progressively controllably reduced and cracks at the extremities of said concrete element due to said prestressing are minimized.

3. A process as claimed in claim 1, wherein the casting of the element comprises casting one end section first, and casting at least one adjacent section subsequently.

4. The process of claim l further comprising the additional steps of casting a third section of concrete about the tendons longitudinally adjacent to said second cast section in continuation thereof, allowing said concrete sections to further harden and further decreasing the tensioning of the tendons by a third predetermined decrement. after said third section has hardened enough to retain an initial prestress of said third decrement, said second section has hardened enough to retain a first subsequent prestress of the sum of said second and third decrements and said first section has hardened enough to retain a second subsequent prestress of the sum of said first, second and third decrements.

S. The process of claim 4 wherein said third decrement tensioning decreasing step comprises totally releasing the tension on the tendons, said first, second and third section 8 prestress being the ultimate progressively distributed prestresses for said concrete element.

6. The process of claim 4 wherein the casting of the element comprises casting one central section as said first section, subsequently casting two adjacent sections flanking said central section as continuations thereof as said second section and then subsequently casting two adjacent sections anking said second section adjacent sections as continuations thereof as said third section, said central section comprising the zone of maximum bending in said prestressed concrete element.

vReferences Cited UNITED STATES PATENTS 3,217,375 11/1965 Knnard 264-Layers Digest ROBERT F. WHITE, Primary Examiner T. P. PAVELKO, Assistant Examiner U.S. Cl. X.R. 

